1. Field of the Invention
The invention relates to a process for reducing unwanted specific electrochemical conversion in rechargeable batteries.
2. Description of Background and Other Information
Storage of energy in its varied forms is a problem which has been given primary attention in technology.
Thermal energy can be stored by heat accumulators having temperatures above or below the normal temperature. Such storage requires that these energy accumulators have reservoirs with thermal insulation.
Latent heat accumulators can also be used for storing thermal energy. In this case, chemical elements or compounds are stored, and insulated against their environment, just above or below their conversion temperature. When a threshold temperature is exceeded in either direction, an additional excess or shortage of thermal energy is released, so that either a lowering or raising of the temperature of the media with respect to the ambient temperature is performed.
As is well known, kinetic energy can be generated by raising the potential energy of energy-storing media and converting the same. As a typical example, storage power plants having pump-fed power storage plants, wherein during times of an excess of electrical energy, the pumps are used to lift water from a potentially lower energy level to a potentially higher energy level. This potential difference as a rule is realized by pumping water from a lower lake level to a higher lake level, so that in times of need for energy the potential energy of the water can be made available as electrical energy by conversion by means of turbines and generators.
Although the storage of energy as electrical energy theoretically has a high degree of efficiency when converted to light, and particularly, to heat; in practice, large technical conversions have encountered a great number of difficulties. For example, the conversion of electrical energy through electrolysis of water to form hydrogen and oxygen, for storage in this form and then reconverting to obtain electrical energy by means of combustion elements, wherein a recombination of hydrogen and oxygen takes place, has not been possible so far on a large technical scale with an appropriate degree of efficiency. Other experimental conversions, with appropriate pairings such as sodium and sulfur, have also failed on a large technical scale, because either high temperatures are required to achieve the appropriate energy density and therefore current output, or the respective devices, because of their operating conditions (e.g., increased temperatures) have only made possible a short service life or low efficiency.
Electrochemical energy reservoirs also have the disadvantage that, on the one hand, their storage capabilities are relatively poor, or that during storage, chemical and electrochemical discharge processes, which are difficult to control, occur, particularly with multi-cell batteries or accumulators.
It is basically possible to distinguish between two different reservoir types in the storage of electrochemical energy. In one reservoir type, the chemical element or chemical compound, the potential energy of which was increased, is placed into or against the electrode. This requires a bipolar electrode. In a second reservoir type, one or two electrochemically effective elements or materials are not placed against or into the electrode, but, instead are placed in their own accumulator. A fluid electrolyte is used for transport, and storage takes place in the reservoirs for anodically or cathodically effective substances. A particularly advantageous storage of electrical energy by means of a circulating electrolyte consists in that the substance acting on the cathode, i.e. a metal (e.g., zinc), is deposited on the cathodically effective surface of the electrode. The element deposited on the anode is guided into a reservoir, wherein a uniform activity of the element is made possible by means of a complex (e.g., an ammonia complex for the bromine molecule), which is only conditionally soluble in the electrolyte (e.g., an aqueous electrolyte).
The principles for storing energy, as discussed above, have been known for a long time, but in practice, the efficiency and the service life of the respective electrochemical reservoirs have been unacceptable, and have consequently hampered their broad employment.
A phenomenon of the electrochemical reservoirs which has received little attention is the loss of potential chemical energy when not in use.
In cases where the phenomenon has been addressed, it is known from EP 0 168 377 A1, to prevent the stray currents, which in a zinc-bromine battery with a circulating electrolyte, run between the individual half-cells when it is idle, i.e. during the period in which current is neither removed from nor supplied to the battery. Such prevention is approached by stopping the second-order electrical connections, and disrupting the flows to and from the individual anode or cathode chambers. Either blocking elements or an appropriate arrangement of the connections are provided for disrupting the flows. Thus, airing of the chamber or chambers can be conducted during which the electrolyte in the lines can be stopped, or replacement of the electrolyte can be accomplished.
However, the loss of potential chemical or electrical energy does not occur only because of routing the flow by means of the supply or draining of electrolyte fluids. Additionally, there is the possibility that the individual cells take care of mutual charge and discharge to establish homogeneous charge equilibrium therebetween. Thus, mutual charging and discharging takes place with, for example, parallel connected batteries or cell packages which have, for example, a plurality of series-connected electrodes, in particular bipolar electrodes.
With the cooperation of diaphragms, this charging process can result in a deposit of metal cathodically on the anode, a phenomenon previously unknown up to now. Consequently, after a predetermined idle period, it becomes practically impossible to draw current from the battery, because it is necessary to first remove the zinc coating from the anode, for example, in order to result in a half cell capable of output.
A process for the specific electrochemical conversion in galvanic cells, in particular a zinc-bromine battery, is known from EP 0 434 659 A2, wherein cell packages or batteries are connected in parallel during charging for better energy utilization, and wherein during discharging, i.e. taking out of current, a switch into series takes place.